Casting has long been a passion of mine. It has aidedme in my work as a model maker and miniaturehobbyist for many years and am sure for many more.Casting is the process of creating duplicates of a singlemaster in your chosen medium and creating anidentical copy in a fraction of the time taken tomanufacture the original or produce many copies.I write this report more as a how to in the hope otherswho read it can pick up on the techniques, tricks andnuances I have learned and am still learning.
The first step in creating a cast is selecting orproducing the item or items you wish to duplicate.Depending what your original (or master) is madefrom and its general shape will determine the bestcasting method used also you will have to decidewhat the duplicates will be cast from, will theyneed to be flexible or solid, hard wearing or shortlived.During this report I will be using a few differentthings for examples.
Knowing what you can produce a mold from and itsinherent properties can be the key to a successful cast. Ifyou make a one-piece mold out of plaster and themaster has an undercut then it’s a failure, where as ifyou make the same mold from the same material butuse a two-part mold then it could be a success.
Molds can be made from a wide range ofmaterials, the most commonly used is siliconedue to its ability to capture fine detail, bend andstretch to allow undercuts to be released, thelife of the mold and the range of thicknesses(Shore). It is not the cheapest medium out therebut if you plan on making many high-endcopies it’s the one to go for.
Plaster is perhaps the cheapest to get your hands on. Itseasy to use and can catch some good detail, the moldswill last for some time if looked after properly i.e. notdropped. The drawbacks to using plaster are that it cantake some time to dry if a particularly wet batch hasbeen made, it is rigid when set so planning your moldwill be essential, it is also harder to remove trapped airfrom in the poring stage unlike silicone.
Resin can be a very good way to make a mold from ifyou are familiar with it. Gellcoat is a thicker type ofresin is lightly brushed on to the master being carefulnot to leave air bubbles trapped. Once it starts tosolidify (or cure) it is given a reinforcement of fiberglassmatting. This method is good to produce a mold formany castings, as it is robust and easy to repair ifsomething should happen to it. It takes very littlelooking after in terms of storage and will outlast mostmolds. The down side is it is a messy process andspecial safety gear is required throughout most of theprocess, it can be very time consuming and is onlycheap if you buy everything in bulk or compare it withsilicone prices.
Latex is a nice cheap easy way to start of a mold. Itcomes as a liquid but has the option of thickening anddoes not require excessive safety gear, but does tend tosmell extremely bad. Set latex is very strong and canbend and stretch better than silicone but it has theunpleasant habit of shrinking. Unfortunately latex can’tbe pored like plaster or silicone and left to set but has tobe applied in layers and allowed to dry which adds on alot of time over all.
A cast can be made from a great many different materials, fromextremely solid concrete and metal to soft expanding foam. Eachmaterial has its own particular quirks that have to be understoodand in some cases overcome to produce a good cast.We’ll start with resin as it is the most widely used.Resin is an exothermic material that requires a kicker to set it ofallowing it to set (methol ethol ketone peroxide). Exothermic is thereaction where heat builds due to a chemical reaction, this is both ahelp and a hindrance as heat is required to allow the resin tosolidify but too much heat and it could crack and fracture. Duringthe setting as the resin cools it can shrink and distort, some cheaperresins do this more than the expensive ones. Even if all you canafford is the cheap stuff help is at hand, in the form of fillerpowders. Powders allow the excess heat to be redirected anddissipated preventing cracking, it also bulks out the batch savingmoney on resin and hinders distortion. Filler powders come inmany different types metal, marble, chalk and even glass bubbles.
Making a cast in foam can be an interesting choice.Some foam solidifies hard when set and others endup very soft and pliable. Some of the slightly moreexpensive ones are self-skinning; meaning as thefoam expands (being made of bubbles) it will pressup against the mold walls and create a smoothsurface. Foam that is not self skinning will need athin brush of latex over the inside of the mold thatis left to dry then foam pored in.Metal is a totally different substance to work withso we shall go into further detail about that later.
During the coming examples I will be telling you to mix or add certainmaterials, so you don’t get confused we will cover them here first.Casting materials come in many different forms and from many differentplaces but will do the same job as their counterparts, with this in mind Icant tell you how to mix everything precisely. I will press the point ofalways read the bottle instructions first. Don’t assume the mix is the sameas the last one you may have used. Always double check labels.As a general rule resin when mixed is a 100% to a 3% hardener. This is asimple way of working it out if you don’t have a calculator to hand.(Example. A cup is holding 600 grams of resin; we need 3%, which is 18grams. 600 / 100 = 6 x 3 = 18)Gel-coat takes around 5% and silicone about 5% also. Some resins take a50/50 mix, that’s why you should always read the label. Other materialssuch as thixotropic agents (for thickening latex, silicone etc.) are added toeye, just add small amounts until you reach the desired consistency (one acertain amount is added to will not thicken any more so don’t go nuts)
There are a few way to make a mold, we will becovering single part and two part molds. Asstated earlier choosing the correct type of moldcan spell success or disaster for you’re casting.The first obstacle we shall look at is anundercut. These are parts of the master that willprevent the new cast from coming out the molddue to it being too large to exit the poring holeor hooking the mold stopping a smooth pull.Below are two molds with different shapes setinside. Each is a plaster mold and a resin cast.
Example 1 shows how the pyramid shape withits large poring base has no undercuts and socan be pulled without any trouble.Example 2 shows how a cast can become stuckif not planed correctly.
There are three simple ways to amend thetrapped cast problem. Firstly is to make themold from a block of silicone, allowing the solidresin shape to be manipulated out. Secondlywould be to make it a two part mold allowing itto be parted releasing the cast, this methodmean’s we could still use plaster for the mold(example A). Thirdly we could change theposition of the master if no detail will be lost(example B) Example A Example B
In this section we will go step by step onconstructing a two-part mold from silicon.There are two ways to make one, depending onthe size and complexity of the master willdetermine which would be more economicaland feasible to use.For this example we will be using a piece of aresin kit. I chose this piece because of itsparticularly troublesome shape. For this pieceboxing up the master is the best option.
Step 1.Fabricate a container that is taller than the pieceyou are casting. Half fill it with oil-based clay,do not use air-drying, as it will dry out whilethe silicon sets, potentially trapping the master.I used plasticine because it was cheaper to getmy hands on (clay is easier to work with). Nowremove enough clay so that the master will sitsnugly half way in. Make sure the mold surfaceis smooth and a good connection is made withthe walls and the master.
Step 2Now we need to add the keys. Keys are locating points used thenjoining the mold back together to insure a good fit with nostepping. Keys can be made from almost anything that makes animprint, the end of brushes is a common one to use, and as long itcreates a hole that can be filled it should do. Once this is done weneed to spray some mold release over everything the silicone willcome in contact will, If you don’t have any mold release spray,petroleum jelly (Vaseline) will work just fine, only problem with itis its almost impossible to apply it totally smooth and silicon picksup ALL the detail. With our mold prepped were ready to pore thebase.
Step 3Anyway once the silicone is mixed ideally you shouldput it in a de-gassing chamber unfortunately noteveryone has one. So simply bounce the cup lightly of atable top repeatedly and watch the trapped air rise tothe surface. Don’t do this for to long as the siliconecould start to set. Pore in from a single point allowingall air to be pushed out of the way by the wave ofsilicone. One the master has been covered de-gas it aswe did previously when it was in the cup just to makesure there was no air in the mold. Wait for the siliconeto fully solidify before turning it over.
Remove the remaining clay being careful not to disturb the seals. Ifthere has not been any poring points added to the master beforethis stage they need to be fabricated now. Anything that will act asa channel to allow the resin to be pored through will suffice. Also ifthere are any areas where air can’t escape a smaller channel can beplaced.Once this is done apply more mold release and repeat step 3 to create the top.Having applied the mold release the two silicone part’s will not havebonded together and with a little teasing should split nicely.
This next method is used when dealing withlarger items where it is not practical or possibleto box up your master.We will be using a person (life casting) for ourexample in this one, as it is the main way to casta head. For simplicity sake we will imaginewere casting a cast of a person (a resin head andshoulders). So we don’t have to emphasize onthe health of our model, also things are done alittle differently but we want to focus on thecasting method for inanimate objects for now.
Step1.Again we will need oil based clay. Roll it out into long thin stripsabout 5-10 mm thick. Look at the head and imagine or draw a linefrom the base of the shoulder past the ear over the top of the headand down to the other shoulder making sure it is at the highestpoint. This will prevent any undercuts. This line is you’re guideline, starting with the back build a wall (flange) round the headmaking sure the clay is now covering the line (it will be on the faceside). Smooth it out and make sure there are no gaps between theclay and the head, once this is done make keys all around.
Step2.Lye the head down so when the silicon isapplied gravity wont take it to badly. Give acoat of mold release and make up a batch ofsilicone with some thixotropic agent through itto make it less runny. Paint on a thin layer tomake sure it gets into all the creases and detail.Once this is done layer on the remainingsilicone evenly.
Step3.Once the silicone has solidified it will need to bereinforced by a back layer of fiberglass orplaster-bandage it really doesn’t make to muchdifference except the rigidity in the long run.Make sure that the fiberglass or bandage createsa lip, this will be important later. Flip the headwhen it’s hard and remove all the clay. Repeatsteps 2 making sure the reinforcement layermeets up with the first creating a seal.
Step4Drill securing holes around the flange bigenough for bolts to go though. These will holdthe two molds together whilst casting.Tease the mold apart and remove the master,now were ready to cast into it with whateveryou wish.